1. Field of the Invention
The present invention relates to an oblique projection optical system, for example, to an oblique projection optical system that is incorporated in an image projection apparatus, which has a display device such as a digital micromirror device and a liquid crystal display device, enlarges an image formed on a display device surface and obliquely projects it on a screen surface.
2. Description of Related Art
In recent years, the demand for wide-angle projection in a projector is increasing. And not only in use of rear projection but also in use of front projection, wide-angle projectors that use mirrors in a projection optical system have been developed one after another. Using mirrors in a projection optical system makes it possible to achieve wide-angle projection that has been impossible with only a refraction lens. Accordingly, various types of projection optical systems that include mirrors have been proposed. For example, the Patent Document 1 proposes an oblique projection optical system that has a refraction lens, a concave mirror, and a convex mirror.
Patent Document 1: US2007/184368A1
In a case of a projection optical system for front projection, not only a simple focus mechanism is required but also more compactness of the projection optical system is required compared with that for rear projection. However, in the oblique projection optical system described in the Patent Document 1, because the convex mirror's optical power is weak, the mirror size is large. Besides, because the rotationally symmetric coaxial refraction lens group is relatively long, the lens diameter on the enlargement side is large. Accordingly, it is difficult to achieve compactness of the projection optical system. Besides, in the oblique projection optical system described in the Patent Document 1, because the distance between the concave mirror and the convex mirror is short, aberration correction is inefficient. In addition, because there are only two reflection surfaces that are disposed as optical surfaces in the vicinity of and after the intermediate image, the burden of aberration correction on the reflection surfaces is heavy, and it is concerned that error sensitivity and size would increase.